Animal viruses provide unique and easily identifiable biological probes with which to study cellular processes. Such diverse processes as the entry of biologically active macromolecules into cells, the regulation of nucleic acid synthesis, and the functional activities of subcellular components (messenger RNA, ribosomes, membranes) in cellular physiology have been fruitfully studied using animal viruses. The information derived from such studies has added significantly to our understanding of both animal viruses and the cells they parasitize. Recently, the techniques of molecular virology, cell biology and immunology have been used to explore cellular and humoral mechanisms of host defense against viral pathogens. Such studies have demonstrated the importance of mononuclear leukocytes in host defense and have underscored the variety of mechanisms by which viral infections may be eradicated. The objectives of this research are to obtain fundamental information about the structure and function of externally disposed membrane receptors of mononuclear phagocytes, and the mechanisms by which these receptors govern the uptake and intracellular fate of animal viruses and other particulate materials.